Fix issue of roster data being split across multiple rosters.

Resolved by always normalizing JIDs to bare form, regardless of if they
are JID objects or strings.

Also simplified related code to prefer use of JID objects instead of
strings so they don't need to be parsed multiple times.

sleekxmpp/xmlstream/jid.py

@@ -143,3 +143,6 @@ class JID(object):
     def __hash__(self):
         """Hash a JID based on the string version of its full JID."""
         return hash(self.full)
+
+    def __copy__(self):
+        return JID(self.jid)